1. Field of the Invention
The present invention relates to a semiconductor laser having an externally coupled resonator with collimation optics between the laser diode and the resonator.
2. Description of the Prior Art
Semiconductor lasers which have an emission spectrum of only one longitudinal mode are required for use in optical broadband transmission. Although presently available diodes are in many instances one-mode, or single-mode, during CW operation for unchopped radiation, the known laser diodes operate in a multi-mode fashion when coupled to an optical fiber or when modulated.
Dynamically single-mode semiconductor lasers, defined herein as semiconductor lasers which remain in a one-mode operation even when coupled to an optical fiber or when modulated, can be formed of DFB (distributed-feedback) lasers, with DBR (distributed-Bragg-reflector) lasers, of C.sup.3 lasers, or of coupled laser arrays. See the following references for examples:
1. Utaka, K., Akiba S., Sakai, K. and Matsushima, Y., "Room-temperature CW operation of distributed-feedback buried-heterostructure InGaAsP/InP lasers emitting at 1.57 .mu.m", Electron. Lett. 17 (1981), pages 961 through 963. PA1 2. Abe, Y., Kishino, K., Tanbum-Ek, T., Aral, S., Koyama, F., Matsumoto, K., Watanabe, T., and Suematsu, Y., "Room-temperature CW operation of 1.60 .mu.m GaInAsP/InP buried heterostructure integrated laser with butt-jointed-built-in distributed-Bragg-reflection waveguide", Electron. Lett. 18 (1982), pages 410-411. PA1 3. Tsang, W. T., Wolsson, N. A., Linke, R. A., Logan R. A., "1.5 .mu.m wavelength GaInAsP-C.sup.3 lasers single-frequency operation and wideband frequency tuning", Electron. Lett. 19 (1983), pages 415-417. PA1 4. Kappeler, F., Westermeier, H., Gessner, R., Druminski, M., Zschauer, K. H., "High CW power arrays of optically coupled (Ga,Al)As oxide stripe lasers with dc-to-light conversion efficiencies of up to 36%", 9th IEEE Int. Semicond. Laser Conf., August 1984, Rio de Janeiro.
Each of the lasers disclosed in the above-listed references, however, are very difficult to achieve technologically. For example, DBR lasers or DFB lasers have extremely critical format tolerances and C.sup.3 lasers must be provided with the required stability. For laser arrays, a plurality of lasers must be operated in parallel to achieve a DSM (dynamically single-mode) laser. Such operation, however, requires high power consumption and, as a result, heat dissipation becomes critical.
A further possibility for achieving dynamically single-mode semiconductor lasers is to couple an external resonator to a one-mode laser diode, such as a conventional BH laser diode. A dynamically one-mode semiconductor laser is known from Liou, U. Y., Granlund, S. W., Swan, C. B., "Single-longitudinalmode operation of GRIN external coupled-cavity semiconductor lasers", Opt. Fiber Comm., January 1984, New Orleans. The disclosed embodiment of the laser corresponds to a dynamically one-mode semiconductor laser of the species initially cited. To form such a laser, collimator optics are used, formed of a section of gradient fiber having a quarter pitch length. The end face facing the light exit face of the laser diode is antireflective so that the mirrored opposite end face forms an external resonator with the laser mirror. The effective resonator length is defined by the fiber perimeters and more specifically, by the numerical aperture and fiber radius, and can be changed to only a limited degree by a variation of the fiber length.
Very short resonators which can be required in some instances have a length of between 100 .mu.m and 200 .mu.m which cannot be produced with normally obtainalle fiber perimeters or are very difficult to manufacture therewith. In the reference Voumard, C., Salathe, R., and Weber, I., "Single-mode operation of diode lasers coupled to hemispherical resonators", Opt. Commum. 13 (1975), pages 130-132 is disclosed a dynamically single-mode semiconductor laser in the form of a laser having a coupled external resonator in the form of a spherical concave mirror. The radius of the mirror is in a range of between 50 .mu.m and 200 .mu.m. The manufacture of such small mirror radii of adequate quality and precision is extremely difficult.